1. Field of the Invention
The present invention relates to gas flow modulators and is useful particularly, but not exclusively, for regulating the gas supplies of heating appliances for domestic use and recreational vehicles.
2. Background Art
In the design and manufacture of gas appliances such as furnaces, water heaters, fireplaces and the like, it is often desirable to be able to regulate the flow of a gas to a burner in order to achieve certain desired operational parameters; for example, heat output, output air temperature and/or output water temperature.
In the past, flow control of gas to a burner has been limited to a status of either on or off and without an intermediate gas flow status. A problem with this type of system is that the object heated by the burner, typically a heat exchanger, experiences temperature extremes which in turn subject the same to thermal stresses so as to shorten its period of useful life. Additionally, this type of system fails to conserve energy in that heat loss occurs in the period of time when the gas is not flowing so as to allow the system to cool off rather than maintaining its proper temperature. The cooling off of the system may undershoot the desired low-end temperature, such undershooting representing an amount of heat loss which must then be offset by consumption of enough gas to re-heat the system to an acceptable temperature range. Also, the system may overshoot the desired high-end temperature during the period when the gas is flowing in order to raise the system to the desired temperature. Such undershooting and overshooting represent heat loss which must then be offset to adjust the system to an acceptable temperature range. Further, the range of environmental comfort is not enhanced but is rather deterred by a greater than desired temperature range occurring from over and under shootings.
Also in the past, regulation of the flow of a gas to a burner has sometimes been accomplished by mechanically closed loop systems, employing a sealed capillary tube containing a heat-expandable fluid which is used to sense the temperature at a predetermined location and to generate a control pressure, which in turn is used to cause a change of gas flow in a specially designed valve.
In the case of demand water heaters, a similar approach of mechanically adjustable valves has been used by sensing the pressure of input water by means of a diaphragm or other pressure-responsive device and generating a control pressure in a fluid which, in turn, is applied to a valve designed to adjust a gas flow to a desired value.
A problem with mechanically adjustable valves is the lack of the automatic adjustability known to electrically controlled systems. The development of electronic sensors and controllers for regulating gas flow to a burner requires a gas flow control function to be effected by the use of electrical signals, rather than mechanical, i.e. pneumatic, signals. Such electrically controlled gas flow valves must also be safeguarded from the potentially dangerous condition of the valve sticking and failing to adjust, which may cause heat loss, excess heat, or system failure in general.